This invention relates generally to arrays of solar cells and, more particularly, to solar cell arrays used on spacecraft. It is, of course, well known that arrays of photo-voltaic cells, referred to as solar cells, provide the principal source of electrical power for spacecraft. Although solar cells have improved in efficiency of power conversion, arrays presenting a very large area are still required to fulfill the power requirements of a typical space vehicle. For most spacecraft missions, solar cell arrays of large area are not fully deployed until an operational orbit has been reached.
Typically, an unmanned spacecraft is first launched into a relatively low earth orbit, and then traverses a transfer orbit to attain its final operational orbit, such as a medium earth orbit or a geosynchronous orbit. Solar panels are stowed for launch in a folded configuration, usually with multiple flat panels hinged together and folded in accordion style. Although some of the panels are needed to provide power during the transfer orbit, they must usually be kept folded to minimize spacecraft inertia. If the panels were to be fully deployed in the transfer orbit, the increase in inertia would make control of the spacecraft more difficult. In the transfer orbit, power demands are lower than in operational orbit because one or more payload modules will not be activated until the operational orbit is attained. In the folded configuration, only one panel, the outermost panel, will normally be exposed to the sun, and the power system is designed such that this one panel has sufficient area to supply the needs of the vehicle while in the transfer orbit. When the panels are fully deployed to form a large array of N flat panel segments, the power generation capability of the array is increased by a factor of N.
If the total power requirements of the spacecraft in operational orbit closely match the power generated by the N fully deployed panels, this arrangement is satisfactory. If, however, the power requirements can be met by something less that an integral number of panels, a significant inefficiency arises. For example, if only two and a half full panels are needed to meet the maximum power requirements, and one full panel is needed for the transfer orbit configuration, then the array will still normally be constructed as three full panels. This is because the solar cells on the outermost panel are fully utilized to meet the transfer orbit power requirements. The innermost panel, in this example, may be made with only half its area covered with active cells, making a total of two and a half panels in the deployed configuration. This three-panel configuration has an unnecessarily high inertia because three full panels are used when less than three are needed.
Ideally, it would be highly desirable to provide a foldable solar array that contained only as many cells, in panels and part panels, as were necessary to meet maximum power requirements, but which would still meet the power requirements of the transfer orbit while in a folded configuration. The present invention meets this goal.
The present invention resides in a shortened solar array having a number of panels hinged together for folding in accordion style, but in which an outermost panel is a partial panel, for better matching of the power generation capacity of the array to the maximum power requirements to be met by the array. The invention may be defined as a foldable solar cell array with minimal inertia, but which still provides sufficient cell area to meet power demands in both deployed and folded configurations. The solar cell foldable array comprises a plurality of panels, including an innermost panel for coupling to a spacecraft, and an outermost panel that is smaller in area than each of the other panels. Each panel has first and second parallel faces and each panel is hinged to the next in succession to permit the panels to be moved between a deployed configuration in which the panels are generally coplanar, and a folded configuration in which the panels are stacked together in a generally parallel relationship. The structure of the invention further includes a segmented array of solar cells, with each segment being mounted on the first face of each of the panels, such that the segments together form a practically continuous array in the deployed configuration, and provide sufficient power to meet the maximum demands of the spacecraft. Critical to the invention is an additional segment of solar cells, mounted on the second face of the panel next to the outermost panel. The additional segment, in the folded configuration, forms a sub-array of solar cells with the segment of cells mounted on the first face of the smaller outermost cell, and this sub-array of cells provides sufficient power in the folded configuration to meet spacecraft power demands in a transfer orbit.
More specifically, the outermost panel has an area p.A, where A is the area of each of the other panels and p is a factor less than unity. The additional segment of cells covers an area (1-p).A of the next-to-outermost panel. The outermost panel and the additional segment of cells together provide a cell area A in the folded configuration.
In one illustrative embodiment of the invention, the number of panels in the foldable array is two. The innermost panel and the panel next to the outermost panel are one and the same in this embodiment.
The invention may also be defined in accordance with the illustrative embodiment, as a foldable solar cell array comprising an inner panel having first and second parallel faces and first and second parallel edges, wherein the first edge is pivotally attachable to a fixed yoke on a spacecraft; and an outer panel having one edge pivotally attached to the second edge of the inner panel. The solar cell array is movable between a folded configuration in which the inner and outer panels are substantially parallel with each other and a deployed configuration in which the inner and outer panels are substantially coplanar. The structure further includes a segmented array of solar cells, including a first segment installed over the entire first face of the inner panel and a second segment installed over one entire face of the outer panel. The first and second segments form a practically continuous array in the deployed configuration, and provide sufficient power to meet the demands of the spacecraft in an operational orbit. The outer panel is smaller in area than the inner panel, to minimize total mass and inertia. The structure further comprises an additional segment of solar cells installed on the second face of the inner panel. The additional segment of solar cells on the inner panel and the second segment of cells on the outer panel together form a practically continuous sub-array in the folded configuration, and this sub-array of solar cells provides sufficient power to meet spacecraft requirements in a transfer orbit, without deploying the array.
It will be appreciated from the foregoing that the present invention represents a significant advance in the field of foldable solar arrays. In particular, the invention provides a technique for shortening the outer panel of an array without sacrificing power generation capability when the array is in its folded configuration. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings.